1. Field of the Invention
The present invention relates to a surface acoustic wave convolver for obtaining convolution outputs utilizing non-linear interaction of plural surface acoustic waves.
2. Related Background Art
The surface acoustic wave convolvers are increasing their importance in recent years as a key device in the spread spectrum communication. Also they are actively developed for various applications as real-time signal processing devices.
FIG. 1 is a schematic plan view showing an example of such a conventional surface acoustic wave convolver.
On a piezoelectric substrate 1, there are provided a pair of comb electrodes 2 and a central electrode 3. The comb electrodes 2 are used for generating surface acoustic wave signals, and the central electrode 3 serves to cause propagation of said signals in mutually opposite directions and to obtain an output signal.
When one of said comb electrode 2 is given a signal F(t)exp(j.omega.t) while the other is given a signal G(t)exp(j.omega.t), two surface acoustic waves: EQU F(t-x/v)exp[j.omega.(t-x/v)] (1a) EQU G(t-(L-x)/v)exp[j.omega.(t-(L-x)/v)] (1b)
propagate in mutually opposite directions along the surface of the piezoelectric substrate 1, wherein v is the velocity of said surface acoustic wave, and L is the length of the central electrode 3.
On the path of said propagation, a component of product of said surface acoustic waves is generated by the non-linear effect, and is integrated over the central electrode 3 as the output signal. Said output signal H(t) can be represented by: ##EQU1## wherein .alpha. is a proportional coefficient.
Thus a convolution signal of two signals F(t) and G(t) can be obtained from the central electrode 3.
However, since such structure is unable to provide sufficient efficiency, there is proposed a surface acoustic wave convolver of the structure shown in FIG. 2 (Nakagawa et al., Journal of Electronic Communication Association '86/2, Vol. J69-c, No. 2, pp 190-198).
On a piezoelectric substrate 1 there are provided a pair of input comb electrodes 2 and an output comb electrode 4. Also on said substrate there are provided waveguides 3-1-3-N between said input comb electrodes 2.
When one of said comb electrodes 2 is given a signal F(t)exp(j.omega.t) while the other is given a signal G(t)exp(j.omega.t), the generated surface acoustic waves propagate in mutually opposite directions along the waveguides 3-1-3-N, thereby generating a convolution signal represented by the equation (2) on each propagation path, due to the non-linear effect of the piezoelectric substrate 1.
These signals generate, in a direction perpendicular to the wave guide paths 3-1-3-N, a surface acoustic wave which is converted by the output comb electrode 4 into an electric convolution signal.
However, in such conventional structure, the surface acoustic wave generated by a comb electrode 2 and transmitted through the wave guide paths 3-1-3-N is reflected upon reaching the other comb electrode 2 and overlaps with the surface acoustic wave propagating in the normal direction to cause so-called self convolution. Consequently the conventional surface acoustic wave convolvers are associated with a drawback that the unnecessary signal resulting from self convolution overlaps the desired convolution signal.
In addition the conventional structures cannot be satisfactory in terms of the efficiency.